Microphone



June l0, 1947. R, VEFRMEULEN 2,421,820

MICROPHONE Filed March 25, 1943 ATTORNEY Patented June 10, d 1947 Roelof Vermeulen, Eindhoven, signor to Hartford National Hartford, Conn., as trustee 1943, Serial No. 480,191 ds March 7, 1941 Company,

Application March 23,

In the Netherlan 9 Claims.

This invention relates to microphone-s and, more1 particularly. to, microphones` for use in the case ci a high` noise level. These microphones consist of a plurality of electrically interconnected elementsfor converting -air vibrations into electric; voltage. variations.

Known microphones which are utilized in spaceshavinga high noise level and which have for their purpose toY transmit the useful sound and` topick up as little parasitic noise as possible, consist.- of two sound-responsive elements, each havinga unidirectional characteristic, whose irresponsive sides,` are placed against one another and which are electrically connected to one another in opposite senses. The. parasitic noise is picked up by the two elements in the same measure but, owing to the electrically opposite connection., it is` not further amplified. The sound tobe transmitted can substantially reach only one. of the microphones and is further amplified. The drawback` of such installations is that-the screening of the other microphones against reflecting sound waves must be rather complete. since else the useful sound would be transmitted only ieebly. Moreover, the output energy of thesev microphones is `only half the energy which can be. transmitted with the use of two `microphones. since one of the two microphones. does not transmit useful sound.

'Ihe invention has for its object to provide a microphone which consists of one or more elements which transmit all of the useful sound and which4 jointly suppress parasitic noises. u

According to the invention, the microphone consists of a space surrounded 'by an element or byv a. plurality of elements provided around said Space which are capable of converting pressure gradients into electrical oscillations and which arel so. arranged and interconnected that the acoustic. oscillations coming from outside the space. and acting` on these elements generate therein electric voltages which substantially neutralize one` `another whereas the acoustic oscillations. coming from within the space and acting on. these.. elements generate therein voltages which amplify or add to each other.

As isv well-known, pressure gradient or velocity microphones are responsive to` sound waves which have. a velocity component perpendicular to the plane of the microphone. Such microphones consequentlyhave their maximum responsiveness in twoopposite. directions. With the microphoneaccordng to the invention, one of these directions is turned towards the enclosed space.

Since the various elements are Connected to Netherlands, as,- Bank and Trust each other, the alternating voltages generated either amplify or counteract each other. For sound waves coming from a source of sound outside the space enclosed by the microphone, the pressure gradients of tall the elements or of all parts oi the element are equal". With a proper connection of the elements, the voltages set up thereby neutralize each other. If, however,A the sound cornes from a point located within the enclosed space, the alternating voltages generated add to each other in the case of the same circuit-arrangement.

The microphone may consist of a single ribbon microphone having the shape of a closed ring.` lt is also possible to arrange a plurality ci ribbon microphones symmetrically around la space, said microphones being connected in series in this case. In both cases, means are provided for bringing the sound to be transmitted into the enclosed space. They may consist, for example, of a sound-conducting funnel. It is also possible to leave the space enclosed by the microphone open on both sides so that the mouth of the speaker may .be put into this space.

The invention will be explained more fully with reference to a few embodiments thereof shown by way of example.

The figures of the accompanying drawing represent three embodiments in longitudinal and in transverse section.

Figs. l land 2 show an annular microphone which encloses a space I0. A, circular conducting ribbon Il is stretched between two likewise annular pole shoes l2 and I 3 in a magnetic field. With the aid 'of a plurality of insulated pins I4, the ribbon is supported in the annular space. The two ends of the ribbon are provided with connecting cords I5 and I6. For the purpose of generating. a magnetic eld there are present four magnets Il which are clamped between the two pole shoes I 2 aand I3. By means of four bolts l 8 the pole shoes are kept assembled and, in addition, the magnets are clamped. The whole of it is surrounded by an annular housing I9 oi largemeshed copper gauze in order to protect the microphone `against mechanical damage. The meshes :are chosen of such width that there is no question of any sound-screening effect.. With the aid of a funnel 2c, the sound is led into the space lil.

Parasitic noises originating from the surrounding space may reach the microphones on all sides since the open housing I 9 allows the sound to pass without any impediment. These parasitic noises may reach :all parts of the ribbon II and impart thereto an amplitude whose phases are equal in the absolute sense for all parts. In two parts of the ribbon I l, which are located opposite one another, the one part consequently moves inwards at a given instant land the opposite part moves outwards. The voltages thus set up in these parts have opposite directions and compensate each other to such an extent that no voltage occurs at the connected cords l5 and l. The microphone is consequently irresponsive to sounds coming from the outside surroundings. Ii, however, the sound waves are brought into the space l and if they move radially outward from this space to the ribbon ll, the amplitudes of these oscillations will be o the same phase, reckoned from the middle of the space. The absolute phases of the oscillations of the ribbon consequently have opposite directions; the voltages set up are added. These acoustic oscillations consequently afford electric voltages at the connected cords I and IB.

Figs. 3 and 4 represent one form of construction which consists of four separate ribbon microphones which symmetrically surround a space 2 l. With the aid of four permanent magnets 22 which are all magnetized in the same direction, a mag netic iield is generated between four sets of pole pieces 23, 2li, 25 and 26. Between each of these pole pieces, is located a conducting ribbon 21, 28, 29, 33. These four ribbons are electrically connected in series; the voltage is led away by the cords 3| and 32. The operation completely corresponds to that described with reference to Figs. 1 and 2. The ribbons 21 and 29 generate for sounds originating from the surroundings electrically oppositely connected voltages as do the ribbons 23 and 30 which are located opposite one another. For acoustic oscillations originating from the sound space 2|, the voltages are, however, connected in series successively behind one another. The sum of these voltages may be taken from the wires 3l and 32.

The area of the aperture 2l has such dimensions that the mouth opening of the speaker may just be introduced into the said space so that the sound in the space 2l itself is transmitted to the air. In this form, the microphone is suitable for pilots, guides, and the like. The frequency characteristics of two ribbons located opposite one another are equal, i. e., the dimensions and the compositions of and the voltages in two oppositely located ribbons are the same so that with the same absolute phase also throughout the whole range of frequencies, the alternating voltages neutralize each other. A suitable direct current voltage is applied to the ribbon circuit so as to stretch the ribbons, the natural frequency of each ribbon is then such that the range of frequencies below a predetermined limit is substantially not reproduced, the reproduction of eX- cessive low tones by which speech is rendered unintelligible, being thus avoided. This natural frequency is about 800 cycles per second.

Figs. 5 and 6 represent a very simple form of construction of the microphone. The sound-responsive elements consist of two separate ribbons 33 and 3l! which are provided on either side of the space 35. This space 35 has such dimensions that the mouth of the speaker may be brought between the two microphone elements and brings the sound into the space 35. The ribbons 33 and 33 are connected in series. Each ribbon is located in a separate magnetic field, viz., the ribbon 33 between the pairs of pole shoes 33 with the magnets 3l and the ribbon 34 between the pairs of pole shoes 38 and the magnets 39. The two elements are mechanically kept assembled by two sets of copper strips 4U and 4I, respectively which are clamped together with the aid of four bolts '42.

Since the mouth of the speaker is in front of the space 35, that is to say, in the immediate vicinity of the ribbons 33 and 34, it has been found necessary to provide the ribbons with suitable damping in order to damp excessively large amplitudes and, in addition, to prevent blowing against the ribbons due to the respiration. For this purpose, the air gap is covered on both sides at the points where the two ribbons are present, with a thin piece of ne-meshed cloth 43.

What is claimed is:

1. A microphone for use where high noise levels occur, comprising sound responsive elements for converting air vibrations into variations of electrical voltages, said sound responsive elements being arranged to' form a nearly complete enclosed space, a connection lead joined to the ends of said sound responsive elements, a plurality of magnetic poles adjacent said sound responsive elements, means for leading desired sound waves within said space, whereby the acoustic oscillations originating from the outside of said space act upon the sound responsive elements to set up electrical voltages Awhich substantially neutralize each other whereas the acoustic oscillations com-V ing from within said space act upon said elements to set up voltages which add to each other.

2. A microphone comprising a plurality of sound responsive elements ,for converting air vibrations into variations of electrical voltages, said sound responsive elements being arranged to form a nearly complete enclosed space, a connection lead joined to the ends of said sound responsive elements, a plurality of magnetic poles adjacent said sound responsive elements, means including a funnel shaped member for leading desired sound waves within said space, whereby the acoustic oscillations originating from the outside of said space act upon said elements to set up electrical voltages which substantially neutralize each other whereas the acoustic oscillations coming from within said space act on the elements to set up voltages which add to each other.

3. A microphone comprising a plurality of sound responsive elements for converting air vibrations in variations of electrical voltages, said sound responsive elements being connected in series and arranged to form a nearly complete annular space, a plurality of magnetic poles surrounding said sound responsive elements, a connection lead joined to the ends of said sound responsive elements, means for leading desired sound waves within said space, whereby the acoustic oscillations originating from the outside of said space act upon said elements to set up electric voltages which substantially neutralize each other whereas the acoustic oscillations coming from within said space act on the elements to set up voltages which add to each other.

4. A microphone comprising two similar sound responsive elements which are responsive to pressure gradients, two ring-shaped pole shoes, each one of said sound responsive elements being arranged on each side of said pole shoes to form a nearly complete ring-shaped enclosed space, a plurality of magnetic poles adjacent said responsive elements, a connection lead joined to the ends of said sound responsive elements, means for leading desired sound waves within said space, whereby the acoustic oscillations originating from the outside of said space act upon said elements to set up electric voltages which substantially neutralize each other whereas the acoustic oscillations coming from within said space act on the `elements to set up voltages which add to each other.

5. A microphone comprising a plurality of sound responsive elements for converting air vibrations into variations of electrical voltages, said sound responsive elements being connected in series and arranged to form a nearly complete annular space, a plurality of magnetic poles adjacent said responsive elements, means including a funnel shaped member having an aperture which is located adjacent to said enclosed annularl space, said aperture having substantially the same dimensions as the human voice opening for leading desired sound waves within said space, whereby the acoustic oscillations originating from the outside of said space act upon said elements to set up electrical voltages which substantially neutralize each other whereas the acoustic oscillations coming from within said space act on the elements set up voltages which add to each other.

6. A microphone comprising two similar sound responsive elements which are responsive to pressure gradients, two annular pole shoes, each one of said sound responsive elements being arranged on each side of said pole shoes to form a nearly complete annular enclosed space, a plurality oi magnetic poles adjacent said responsive elements means for leading desired sound Waves Within said space, whereby the acoustic oscillations .located symmetrically opposite one another and arranged whereby the responsiveness of said element is below the range of frequencies required for the intelligibility of human voice to the leading desired sound waves within said space and whereby the acoustic oscillations originating from the outside of said space act upon said elements to set up electric voltages which substantially neutralize each other, whereas the acoustic oscillations coming from within said space act on the elements to set up voltages which add to each other.

7. A microphone comprising a plurality of ribbon sound responsive elements for converting air vibrations in variations and electrical voltages, said sound responsive elements having a natural frequency of 800 cycles per second and being connected in series and arranged to form a nearly complete annular space, a plurality of magnetic poles adjacent said sound responsive elements, means for leading desired sound waves within said space, whereby the acoustic oscillations originating from the outside of said space act upon said elements to set up electric voltages which substantially neutralize each other, whereas, the acoustic oscillations coming from within said space act on the elements to set up voltages which add to each other.

8. A microphone comprising a plurality of I `which substantially neutralize each other, whereas the acoustic oscillations coming from Within said space act on the elements to set up voltages which add to each other.

9. A microphone comprising a plurality of ribbon sound responsive elements for converting air vibrations into variations of electrical voltages, two ring-shaped open pole pieces, said ribbon being interposed between said open pole pieces and arranged to form a nearly complete annular space, a plurality of insulated pins for retaining said ribbon secured to said ring-shaped pole pieces, a plurality of magnetic poles adjacent said sound responsive elements, a connection lead joined to the ends of said sound responsive elements, means including a funnel-shaped member for leading desired sound waves within said space whereby the acoustic oscillations originated from the outside of said space act upon said elements to set up electrical voltages which substantially neutralize each other whereas the acoustic oscillations from within said space, act to set up elements on the voltages which add to each other.

ROELOF VERMEULEN.

REFERENCES CITED i The following references a iile of this patent:

UNITED STATES PATENTS e of record in the 

